The present invention relates to a high-yield process for preparing 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid which is useful intermediate in the preparation of cephalosporin antibiotics.
7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid of formula (I) is a precursor of cefpodoxime proxetil, and there have been reported many methods for the preparation thereof starting from 7-aminocephalosphoranic acid(7-ACA) of formula (II). 
For example, 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid has been prepared by the steps of protecting the 7-amino group of 7-ACA with a phenylacetyl group; converting the 3-acetoxy group to a methoxy group by the action of methanol-sodium bicarbonate or methanol-calcium chloride; and removing the protection group (see JP Patent No. 82,192,392 and U.S. Pat. No. 4,482,710). However, this method has the problems that yields obtained are very low (approximately less than 20%) and the process requires multi-steps.
JP Patent No. 84,163,387 teaches a method of preparing 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid by treating 7-aminocephalosporanic acid with methanesulfonic acid-methanol. However, this method also has the problem of low yield (approximately 30%), and the product purity is poor (approximately 30 to 40%) due to the formation of by-products such as lactone or the degradation materials of the xcex2-lactam ring.
Alternatively, 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid may be obtained by reacting 7-ACA in sulfolane with boron trifluoride-methanol (see EP patent No. 204,657), but this method requires the use of gaseous boron trifluoride which is hazardous and difficult to handle.
EP Patent 262,744 discloses a method of preparing 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid by reacting 7-ACA with methanol in the presence of a halide of antimony or zinc. However, this method is hampered by the problem of low yield (approximately 40%) and is not suitable for mass production due to the use of column chromatography for separating final product.
Further, 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid has been prepared by treating 7-aminocephalosporanic acid with boron trifluoride-methanol in the presence of halosulfonic acid or alkylsulfonic acid (See JP Patent No. 88,115,887) or zinc chloride-methanol (See JP Patent No. 89,242,590). These methods give relatively good yields (approximately 60%), but still have the problem of low product purity.
EP patent No. 343,926 teaches a method of preparing 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid by reacting 7-aminocephalosporanic acid with trimethyl borate in sulfolane, in the presence of sulfuric acid and antimony pentachloride. However, this method requires the use of expensive antimony pentachloride as well as 98% trimethyl borate which is difficult to handle.
EP patent No. 485,204 teaches a method for preparing 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid by treating 7-ACA in a solution containing an alkoxysulfonic acid with a trialkyl borate and CH2(OR) 2. However, this method still has to deal with the difficulty of handling 98% trimethyl borate and also suffers from the problem of poor process controllability.
Accordingly, there has existed a need to develop an improved method for preparing 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid.
It is, therefore, an object of the present invention to provide a high-yield process for preparing 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid of high purity.
In accordance with the present invention, there is provided a process for preparing 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid of formula (I) which comprises reacting 7-aminocephalosporanic acid of formula (II) with a trimethyl borate-methanol azeotropic mixture in the presence of methanesulfonic acid: 
The compound of formula (I) may be prepared by reacting 7-aminocephalosporanic acid of formula (II) with a trimethyl borate-methanol azeotropic mixture in the presence of methanesulfonic acid.
In the inventive process, 7-aminocephalosporanic acid may be reacted with methanesulfonic acid and a trimethyl borate-methanol azeotropic mixture in the absence of any added solvent. Methanesulfonic acid may be used in an amount ranging from 5 to 20 equivalents, preferably from 10 to 15 equivalents based on 7-aminocephalosporanic acid.
A trimethyl borate-methanol azeotropic mixture according to the present invention is composed of 70% trimethyl borate and 30% methanol which can be easily formed by an azeotropic distillation and is also commercially available like any other chemicals.
The amount of the trimethyl borate-methanol azeotropic mixture employed in the present invention corresponds to 2.0 to 5.5 equivalents of trimethyl borate and 3.0 to 7.5 equivalents of methanol, based on the amount of 7-aminocephalosporanic acid used.
In the preferred embodiment, the inventive process for preparing the compound of formula (I) may be conducted by (1) dissolving 7-aminocephalosporanic acid in a mixture of methanesulfonic acid and a portion of the azeotropic mixture; and (2) adding the remaining portion of the azeotropic mixture to the solution obtained in step (1).
In step (2), the portion of the trimethyl borate-methanol azeotropic mixture may be added, either dropwise over a period of for 1.5 to 2 hours or in several divided portions at an interval of 20 to 30 minutes, to inhibit the formation of by-products such as lactone or the degradation materials of the xcex2-lactam ring. The amount of the trimethyl borate-methanol azeotropic mixture employed in step (2) is 1.5 to 4.5 equivalents, preferably from 1.8 to 2.6 equivalents of trimethyl borate, and 2.0 to 6.0 equivalents, preferably from 2.5 to 3.7 equivalents of methanol, based on the amount of 7-aminocephalosporanic acid used.
The above reaction in accordance with the present invention may be performed at a temperature ranging from xe2x88x9210 to 40 xc2x0 C., preferably, from 5 to 15 xc2x0 C. After adding the trimethyl borate-methanol azeotropic mixture, the reaction is conducted for 1 to 3 hours.
After the completion of the reaction, the pH of the resulting solution is adjusted to 3.0 to 3.5 by using a base to obtain 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid in a crystalline form. Exemplary bases that may be used in the present invention include alkali metal formates, acetates, bicarbonates and carbonates.
The method of the present invention is very simple and provides 7-amino-3-methoxymethyl-3-cephem-4-carboxylic acid of 97% or higher purity in a yield of greater than 80%.
The following Examples are intended to further illustrate the present invention without limiting its scope; and the experimental methods used in the Examples can be practiced in accordance with the Reference Examples given herein below, unless otherwise stated.
Further, percentages given below for solid in solid mixture, liquid in liquid, and solid in liquid are on the bases of wt/wt, vol/vol and wt/vol, respectively, unless specifically indicated otherwise.